Diversity of transition pathways in the course of crystallization into ice VII

Abstract

We report various types of pathways emerging in the course of freezing into a high pressure ice VII by large-scale molecular dynamics (MD) simulations at 10 GPa, 425 K. Some trajectories showed an apparently stepwise transition via the unique metastable "phase" in accordance with Ostwald's step rule. The metastable structure was identified as one of the tetrahedrally close-packed structures having the nature of a rotator phase (plastic phase). The unit cell consists of 21 water molecules that has not yet been reported by simulation or by experiments. Structure analysis of numerous trajectories reveals that the nucleation of ice VII easily occurs at the grain boundaries of the metastable embryos, known as epitaxy-mediated transformation in line with Ostwald's step rule. The size of the grain boundaries is thus responsible for the lifetime of the metastable phase. On the other hand, once the embryo of ice VII appears prior to that of the metastable phase in liquid, direct transition to ice VII takes place. We also show that the plastic behavior of the metastable phase is not uniform but is sensitive to the interaction strength of adjacent molecules and the local structure, thus we call "partially plastic ice".

Liquid-liquid separation of aqueous solutions: A molecular dynamics study

“…89,90 12 The most stable solid phase at pressures higher than 10000 bar is either ice VI or ice VII (ignoring hydrogen-ordered phases). In computer simulations using classical water models, however, crystalline structures that have not been prepared experimentally can be more stable than ice VI and VII, 77,[91][92][93][94] probably because of the Lennard-Jones potential used instead of more precise functions such as the exp-6 Buckingham function. 95 Ice T2 is one of such unreal high-pressure ice crystals.…”

Stepwise Homogeneous Melting of Benzene Phase I at High Pressure

“…These results imply that the metastable structure can be a "phase" rather than a transiently appearing fragile structure. Its free energy is likely to be between phase I and liquid benzene, in accordance with Ostwald's step rule [23].…”

“…Five crystalline phases of pure benzene have been reported so far experimentally [7][8][9][10][11][12][13][14], and computational studies predicted many other potential crystalline structures [1,[15][16][17][18]. Although the phase diagram shows the most stable phase at a given condition, there can be rich intermediate phases on the transition pathway from the initial to the finally prevailed phase [19][20][21][22][23]. Ostwald argued that a phase transition can proceed via an intermediate metastable phase due to the reduction in the surface energy of nucleation (Ostwald's step rule) [24] in contrast to the direct nucleation described in the classical nucleation theory [25].…”

Computational Study on Homogeneous Melting of Benzene Phase I

“…After several hundred ps, the ordered crystal structure spontaneously collapses with increasing potential energy. If there is a transient metastable state between the crystal and liquid phases, potential energy presents a stepwise change owing to the Ostwald step rule [54]. However, the homogeneous melting of benzene in the 200 melting trajectories observed in our study was always completed without any steps.…”